The invention relates to an arrangement for measuring the distance between a metallic workpiece and a processing tool moved relative to the workpiece, especially for adjusting the distance of acetylene, plasma and laser cutting machines whereby at least one capacitance and/or inductive sensing device facing the surface of the workpiece to be processed is provided to the tool; the electrical properties of which can be affected by reducing or increasing the distance to the workpiece and whereby, furthermore, the sensing device is connected to an electrical circuit to evaluate the changes of the electrical properties of the sensing device.
Such arrangements have been known and have been used for a long time already. Electrodes are used as a sensing device which form a reciprocal distance-proportional capacitance with the workpiece, used as a counter electrode. Inductive sensing devices with a coil are also known which are changed distance-proportional in the inductance value by a metallic workpiece. Such arrangements are, for example, described in British Pat. No. 840,275 (British Oxygen Company, Ltd.), U.S. Pat. No. 3,153,109 (C. F. Steventon), U.S. Pat. No. 3,217,204 (H. M. Nance), U.S. Pat. No. 3,596,050 (G. H. Tikijian) as well as German Published Application No. 1,941,728 (Messer Griesheim GmbH).
The evaluation of the distance-proportional change of the applied sensing capacitance or the sensing inductance is carried out, as a rule, in such a way that the sensor is connected in a resonant circuit as a frequency-determining element. Each distance change leads, therefore, to a mistuning of the resonant circuit. This frequency change can then be converted without problems in a known way, for example, by means of a discriminator into DC signals which can be used for indication and/or distance adjustment.
A more serious disadvantage of the known arrangement is primarily that the sensing arrangements are not only affected by distance changes in processing direction to the workpiece, but that also a lateral approach to the workpiece leads to an affecting of the sensing capacitance or sensing inductance. The known arrangements for distance measuring have found application primarily in installations for cutting metal plates. Processing of workpieces with a considerably wavy surface is not possible with the known arrangements, however, since the lateral effect, in other words, also the effect on the sensor parallel to the directon of movement as a result of a material elevation leads to the fact that the sensor "registers" an approach to the workpiece and the processing tool is, therefore, moved away from the workpiece without the occurrence of an actual distance change to the workpiece in the immediate area below the sensor or the tool.